The invention relates to an electronic ignition with an ignition coil, in which the primary winding lies in a circuit with a transistor controlled by a control signal and a current limiting circuit whereas the secondary winding lies in the circuit of the spark gap.
Conventional automobile ignition devices operate with a mechanical contact, which is opened and closed. This contact lies in the circuit of the primary winding of the ignition coil, which is connected between the poles of the battery. If the contact is closed a current I.sub.pr, flows through the primary winding which corresponds to a magnetic energy, related to the primary winding L.sub.1 of a magnitude EQU E.sub.M =L.sub.1 /2.multidot.I.sub.pr.sup.2
When opening the switch, this energy is released and produces a voltage, with which the spark plug is ignited on the secondary side and thus the magnetic energy may be converted into spark energy.
The switch is opened and closed by the crankshaft of the engine; in a conventional 4-stroke engine the contact has to be opened and closed twice during one rotation of the crankshaft. If the engine runs for example at a speed of 5,000 revolutions per minute then this corresponds to a cycle duration of the ignition contact of 6 ms. At a speed of 2,500 revolutions per minute the cycle duration increases to 12 ms. The ignition coil used has a time constant because of its resistance loss and requires a time of 5 ms for example for the rise in the primary current from 0 to 7.5 A, when there is a battery voltage of 10 V. A further period of 1 ms is provided for breaking down the energy of the ignition spark at a speed of 5,000 rpm. The contact is therefore closed for 5 ms at the said speed, the current through the primary coil rising to the desired value of 7.5 A during this time. The ratio between the contact closing time and the cycle duration is designated as the closure angle. This closure angle s is therefore s=5/6=83.3% in the example stated. This closure angle is between 65 and 85% depending on the number of cylinders of the engine.
If the closure angle given by way of example as s=83.3% is maintained, then with a speed of 2,500 revolutions there is a contact closure time of 10 ms and a contact opening time of 2 ms. These cycle times are doubled when the engine speed is halved again. It is apparent from this that the current through the primary winding of the ignition coil at speeds of less than 5,000 revolutions per minute rises beyond the maximum value of 7.5 A for example which is required for ignition. This is not desirable since the energy in the ignition coil rises with the current and therefore more heat has to be converted. This may lead to the destruction of the ignition system at very low speeds and at fairly high battery voltages.
A current limiting circuit has been proposed for limiting the current to the required maximum value.
Since, in this known system, the energy stored in the coil is only dependent on the level of the current and not on the cycle of the current flow, there is an output loss resulting from the residence time of the primary current at its maximum, which cannot be used to ignite the spark gap. Therefore it is desired to shift the beginning of the increase in current in the primary winding of the ignition coil so that the current reaches its maximum value at the moment of the ignition pulse. However, in order to control this process, it is necessary to derive a variable value from the residence time of the maximum current in the primary winding of the ignition coil, which value may then be used to control the electronic ignition system.